There will be described an example of this type device in the prior art with reference to FIG. 8. Referring to FIG. 8, there is provided a carrier 2 reciprocating along an axis of a platen 1. A ribbon base 3 for detachably retaining a ribbon cassette 10 for the multi-color ribbon is pivotably mounted to the ribbon base 3 by a support shaft 4. A step motor 5 and gears 6, 7 and 8 sequentially driven by the step motor 5 are mounted to the carrier 2. A fan-shaped gear 9 meshing with the gear 8 is fixed to the ribbon base 3. In this prior art device, when rotation of the step motor 5 is transmitted from the gears 6, 7 and 8 to the gear 9, the ribbon base 3 is vertically pivoted with the ribbon cassette 10 in a direction perpendicular to the platen 1, thus arranging a desired color zone of the ribbon in opposed relation with the platen 1.
In the case that the ribbon is a four-color ribbon having the width of 19 mm, that is, a width of 4.75 mm for each color zone, a height of characters is generally 3.2-4.1 mm, and assuming that the height is 4.1 mm, upper and lower margins on both sides of each color zone are only 0.325 mm. Therefore, if vertical deviation of position of the ribbon is created, there will occur mixing of colors. To cope with this problem, a stopper 11 abutable against the ribbon base 3 is mounted to the carrier 2 to determine a reference position of the ribbon cassette 10, and a fixed pulse is applied to the step motor 5 to determine a moving range of the ribbon base 3. Thus, a ribbon selecting position with respect to the platen 1 is controlled.
However, the following problems are included in the prior art. If the printer is initialized under the condition that the ribbon base 3 is located at the reference position, the step motor 5 falls out of step since no slip is generated among the gears 6, 7 and 8 and the gear 9. Accordingly, even if the step motor 5 is stopped, and is driven again by applying a pulse by the predetermined number of steps, deviation of phase upon step out of the step motor 5 cannot be corrected. As a result, the vertical position of the ribbon is deviated to cause mixing of colors.
The aforementioned problem may be eliminated by detecting the phase of the step motor 5 upon step out, and feed-backing the detection signal to a control circuit to correct the input pulse to the step motor 5. However, these measures will cause a great increase in a production cost, and therefore they are disadvantageous in practical use.